This invention relates to corona-resistant resins and films, to electrical insulation systems wherein such corona-resistant resins and films are used, to components of dynamoelectric machines and transformers insulated thereby, and to the machines and transformers incorporating them.
Resin compositions are generally understood to be relatively low-molecular weight materials that, on heating or addition of hardener, are converted to high-molecular weight solids having useful properties. Another general class of polymeric (that is, plastic) materials is understood to be thermoplastic. These thermoplastic materials are generally handled in their high-molecular weight state. Thermoplastic materials exhibit good solubility in solvents, while cured thermosetting resins are insoluble. Many thermoplastic materials also soften and flow when heated, while thermosetting plastics may soften but do not flow when heated. Both cured thermosetting resins and thermoplastic films are employed as dielectric materials. Accordingly, as used herein and in the appended claims the term polymeric material refers to both thermosetting resins and to thermoplastic films.
However, dielectric materials used as insulators for electrical conductors may fail as a result of corona occurring when the conductors and dielectrics are subjected to voltages above the corona starting voltage. This type of failure may occur for example in certain electric motor applications. Corona induced failure is particularly likely when the insulator material is a solid organic polymer. Improved dielectric materials having resistance to corona discharge-induced deterioration would therefore be highly desirable. For some applications, mica-based insulation systems have been used as a solution to the problem, whereby corona resistance is offered by the mica. Because of the poor physical properties inherent in mica, however, this solution has been less than ideal.
Solid, corona-resistant dielectric materials are particularly needed for high-voltage apparatus having open spaces in which corona discharges can occur. This is especially true when the space is over approximately 1 mil in thickness and is located between the conductor and the dielectric, or in the dielectric material itself, or is located between the dielectric and a second dielectric or between the latter and ground. The service life of the dielectric is much shorter when these gaps or spaces are present. This problem is exemplified by dynamoelectric machines such as AC motors in which corona-resistance is essential in wire insulation and use of conventional wire enamel is consequently precluded. Thus, for instance, when the design stress is above the corona-inception threshold and the turn-to-turn or strand-to-strand dielectric strength required exceeds the capability of any known wire enamel which has been degraded by corona activity. The stator coil turn-to-turn corona resistant insulation therefore takes the form of glass-or mica-bonded resinous material as a bulky composite which effectively prevents corona degradation of insulation leading to motor failure in normal use. The glass or mica composite thus occupies space in the core which otherwise could accommodate additional copper and thereby reduce the size of the motor, generator or transformers.
Resins containing a minor amount of organo-metallic compound of either silicon, germanium, tin, lead, phosphorus, arsenic, antimony, bismuth, iron, ruthenium or nickel are disclosed by McKeown (U.S. Pat. No. 3,577,346) as having improved corona resistance. Corona lives of up to four hundred times that of polymers without the organo-metallic additive are disclosed. There is no mention, however, of the use of organosilicates or organoaluminates.
A composition having anti-corona properties is disclosed, by DiGiulio et al, in U.S. Pat. 3,228,883, to consist of a mixture of ethylene-alpha-olefin copolymer, a homo-or copolymer covulcanizable therewith and a non-hygroscopic mineral filler, such as zinc, iron, aluminum or silicon oxide. However, there is no appreciation whatsoever in this patent that the use of submicron-sized alumina or silica particles is necessary to achieve significant improvement in corona resistance. See tables below.
A molded epoxy resin composition which contains alumina and silica is disclosed by Linson, in U.S. Pat. No. 3,645,899, as having good weathering and erosion resistance, but appears to have no particular resistance to corona breakdown.
Epoxy resins containing significant amounts ofreactive organosiloxane derivatives are disclosed by Markovitz in U.S. Pat. Nos. 3,496,139 and 3,519,670. However, these materials are less than ideal since their high reactivity results in a diminished shelf-life, a characteristic often of considerable importance. Moreover, the amine silicones in the U.S. Pat. No. 2,496,139 are polysiloxanes which are made from difunctional and trifunctional silicones, that is, the silicon atoms have either two Si--O bonds and two Si--C bonds, or three Si--O bonds and a single Si--C bond. This is in distinct contrast to the present invention which, as seen below, employs silicates and aluminates, both of which exhibit only Si--O and Al--O bonding.
Epoxy resins containing metal acetylacetonates in amounts below 5% by weight are disclosed in U.S. Pat. No. 3,812,214, but these resins have no corona-resistant properties.
Polymeric resins containing silica and talc as fillers appear to be disclosed in U.S. Pat. 3,742,084 issued June 26, 1973 to Olyphant et al. However, there is no appreciation that subm-icron particle sizes are critical for improved corona resistance when silica is employed.
Likewise, resins containing submicron silica appear to be disclosed in U.S. Pat. No. 4,102,851 issued July 25, 1978 to Luck et al. However, silica is added only as a thixotropic agent and there is no appreciation or concern regarding corona-resistant properties.
Polyethylene resin with various fillers, including alumina and silica, appears to be disclosed in U.S. Pat. No. 2,888,424 issued May 26, 1959 to Precopio et al. But again, properties; the fillers, including such counterproductive materials for corona properties as carbon black, are added only to improve mechanical properties.
Resins containing submicron silica also appear to be disclosed in U.S. Pat. No. 2,697,467 issued Oct. 10, 1972 to Haughney. Like the patent to Luck et al., however, this patent discloses no appreciation or concern for corona-resistant properties.
Curable polyester resin compositions of unsaturated polyester resin and 0.1 to 20 weight percent of an organoaluminate compound are disclosed in U.S. Pat. No. 4,049,748 issued Sept. 30, 1977 to Bailey. Again, however, there is no disclosure of any corona-resistant characteristic and, in fact, the covered products have poor high-temperature dimensional stability because they necessarily contain from 10 to 80 weight percent vinyl resin. This characteristic alone would make these products unsuitable for use in electric motors and similar applications, but they are deficient in the additional respect that they are inherently quite lossy at normal electric motor operating temperatures because of their substantial unsaturated polyester resin content of 20 to 90 weight percent.
Thus, there is a continuing need for corona-resistant materials which are easily fabricated for use as electrical insulation and a further need for additives which can convert dielectric materials susceptible to corona damage to corona-resistant materials. Accordingly, it is the principal object of the present invention to provide a corona-resistant resin, useful in various electrical insulation forms to satisfy these long-felt needs.
Another important object of this invention is to use to best advantage the unique corona resistance of these novel materials in the design and construction of new lines of components of dynamoelectric machines and transformers.